DFS: add dfs_file_flush prototype in dfs_file.h

components/dfs/include/dfs_file.h

@@ -36,6 +36,7 @@ int dfs_file_read(struct dfs_fd *fd, void *buf, rt_size_t len);
 int dfs_file_getdents(struct dfs_fd *fd, struct dirent *dirp, rt_size_t nbytes);
 int dfs_file_unlink(const char *path);
 int dfs_file_write(struct dfs_fd *fd, const void *buf, rt_size_t len);
+int dfs_file_flush(struct dfs_fd *fd);
 int dfs_file_lseek(struct dfs_fd *fd, rt_off_t offset);
 int dfs_file_stat(const char *path, struct stat *buf);
 int dfs_file_rename(const char *oldpath, const char *newpath);